Why does the garden tiger moth have such bright hindwings?

The scarlet-red hindwings of the garden tiger moth are an aposematic (warning) signal directed at predators. Most vertebrate predators — birds, lizards, small mammals — learn to associate vivid red, yellow, or orange with unpleasant or toxic prey. The garden tiger moth is genuinely defended: it contains pyrazines and other chemicals, derived partly from sequestered plant compounds accumulated during the larval stage, that make it unpalatable or toxic to many predators. When at rest with forewings closed, the moth is camouflaged by its brown-and-white pattern; when disturbed, it raises the forewings abruptly to flash the scarlet hindwings — a sudden display intended to startle and warn. This 'flash display' can be combined with the release of a defensive froth from glands on the thorax.

What is the woolly bear caterpillar?

The 'woolly bear' is the caterpillar of the garden tiger moth — a densely hairy larva with long, black-and-rusty-orange bristles covering the entire body, making it one of the most immediately recognisable insect larvae in Europe and North America. The hairs serve a multiple defensive function: they are mildly irritating to the skin and mucous membranes of predators (and humans who handle them incautiously), they make the caterpillar difficult to grip and swallow, and the hairs are loosely attached so they break off and embed in the mouth or eyes of an attacker. The caterpillar feeds on an exceptionally wide variety of low-growing plants — it is one of the most polyphagous of all European moth larvae — and overwinters as a half-grown caterpillar, resuming feeding in spring before pupating in early summer.

Is the garden tiger moth declining?

Yes — the garden tiger moth has shown a significant population decline in the United Kingdom and parts of northern and western Europe since the 1970s. UK Rothamsted moth monitoring data show a decline of approximately 89% between 1968 and 2007 — one of the steepest declines recorded for any common British moth. The causes are not fully established, but climate change is the primary suspect: the species overwinters as a larva, and warmer, wetter winters appear to increase larval mortality through fungal infection, desiccation risk, or disruption of diapause. Paradoxically, the decline is most pronounced in areas with milder winter climates. The species remains common across most of continental Europe, Russia, and much of its North American range, and is assessed as Least Concern globally. The UK decline may therefore represent a range contraction from the western, oceanic margin of its distribution rather than a global trend.

Butterfly Atlas

Arctia caja

Garden Tiger Moth

~2 min

The garden tiger moth (Arctia caja): brown-splashed forewings, scarlet warning hindwings. Its woolly bear caterpillar is a familiar temperate-zone larva.

Key facts

Latin name: Arctia caja
Family: Erebidae
Wingspan: 45–65 mm
Flight season: July — August
Host plants: Stinging nettle (Urtica dioica), Dock (Rumex species), Plantain (Plantago species), Dandelion (Taraxacum officinale), Groundsel (Senecio vulgaris), Many other low-growing herbaceous plants
Conservation status: LCLeast Concern

Appearance

The garden tiger moth is one of the most strikingly patterned moths in the Palearctic, with a colour scheme that operates on two levels: camouflage when at rest, warning display when disturbed.

Forewings: The forewing ground colour is creamy white to warm buff, overlaid with an irregular pattern of dark chocolate-brown markings that are unique to each individual — no two garden tiger moths have identical forewing patterns. The markings are distributed as blotches, bands, and spots that break up the moth's outline against tree bark, lichen-covered stone, and dead vegetation. This individual variation has made the garden tiger moth a model organism in studies of natural selection operating on variable populations.

Hindwings: Entirely hidden when the moth is at rest, the hindwings are vivid scarlet-red (occasionally orange-yellow in rare variants) with four or five large blue-black spots ringed with white. The contrast between the dark forewing and the brilliant hindwing is extreme — when revealed, the hindwing pattern is visually shocking after the camouflage of the forewing.

Body: The abdomen is orange-red, marked with a line of black dots — continuous aposematic warning colouration visible when the hindwings are raised. The thorax is dark brown above, and in some individuals has a distinctive white-spotted pattern. The head and shoulders are dark brown with a white collar.

Wingspan ranges from 45 to 65 mm, with females generally larger than males.

Range

The garden tiger moth is a Holarctic species — its range spans the temperate and boreal zones of both the Old and New Worlds:

Europe: from the British Isles and Scandinavia (to 70°N in Norway) south to the Mediterranean; widespread throughout, though showing signs of decline in oceanic western areas
Asia: across temperate Russia, Siberia, the Caucasus, Central Asia, northern China, Korea, Japan
North America: across Canada and the northern United States, where it was long treated as a separate subspecies (Arctia caja americana) before being synonymised with the nominate

It is absent from the tropics and subtropics, and from true desert and high-arctic zones. In mountains it ascends to approximately 2,000 m.

Life cycle

Egg and early larva

Eggs are laid in large batches of 100–300 on the underside of leaves of the larval host plant in July–August. The female deposits a single mass, often covering a significant area of the leaf underside. Eggs hatch in approximately two weeks, and the tiny first-instar caterpillars initially feed gregariously before dispersing.

The woolly bear caterpillar

The caterpillar is the most conspicuous stage of the life cycle and the most familiar to the general public. Fully grown, it reaches 55–70 mm in length — a substantial caterpillar that is impossible to overlook.

The body is covered with dense, long, mixed black and rusty-orange bristles (setae) arranged in tufts along the length of the body. The head capsule is black and shiny. The dark, hairy appearance gave rise to the "woolly bear" name, shared in North America with several related Arctia species.

Overwintering behaviour: The caterpillar hatches in August or September and feeds for several weeks before the onset of cold weather. It then overwinters as a half-grown larva — hibernating in a sheltered position in dead vegetation, leaf litter, or under stones. In spring, it resumes feeding and grows rapidly through the remaining instars, reaching full size by May or early June.

Diet: The woolly bear caterpillar is highly polyphagous — it will accept an extraordinarily wide range of low-growing herbaceous plants, including nettle (Urtica dioica), dock (Rumex), plantain (Plantago), dandelion (Taraxacum), groundsel (Senecio), comfrey (Symphytum), and many others. This polyphagy makes it one of the few moth caterpillars truly at home in disturbed, weedy habitats — roadside verges, waste ground, and gardens.

Hair irritation: The long setae of the woolly bear are mildly urticating — they can cause skin irritation or eye inflammation in humans who handle the caterpillar carelessly. Predators that attempt to eat the caterpillar encounter these hairs in their mouths and throats, and birds in particular often abandon woolly bear caterpillars after initial contact. Some birds have learned to roll the caterpillar on the ground to remove or mat down the bristles before eating — a behaviour observed in rooks and magpies.

Pupa

The caterpillar pupates in a cocoon constructed partly of its own shed hairs (incorporated into a loose silk structure) among dead vegetation at ground level. The pupal stage lasts 3–4 weeks, producing the adult in July.

Adult

Adults are nocturnal, flying from late July through August and into September in the south of the range. They come readily to artificial light. Adults have functional eyes and use acoustic bat-avoidance behaviour (producing ultrasonic clicks from the tymbal organ on the metathorax to jam bat sonar — a system widespread in Arctiinae). In this, as in their toxicity, they resemble bees and wasps in the effectiveness of their defences.

Adults do not feed — they have vestigial mouthparts insufficient for liquid uptake — and live entirely on larval fat reserves for their brief adult life.

Defence systems

The garden tiger moth has one of the most comprehensive chemical defence systems of any European moth:

Chemical sequestration: the larva accumulates plant-derived compounds (pyrrolizidine alkaloids from Senecio and related plants) that are retained in the adult
De novo synthesis: adults can also produce pyrazines independently, supplementing sequestered compounds
Defensive froth: when seized by a predator, adults discharge a frothy secretion from thoracic glands containing these compounds — often sufficient to cause a bird to drop and reject the moth
Flash display: abrupt revelation of the scarlet hindwing startles predators and reinforces the aposematic signal
Acoustic warning: the tymbal clicks are not only bat-jamming signals but may also serve as acoustic aposematic signals to bats that have learned to associate the clicks with unpalatable prey

Population trends and decline

The garden tiger moth is one of the most intensively monitored moth species in Britain. Rothamsted Insect Survey light-trap data covering 1968–2007 show a decline of approximately 89% in recorded abundance — one of the most severe declines in the dataset for any once-common species.

The causes remain actively debated:

Climate change: warmer, wetter winters appear to destabilise larval overwintering, possibly through increased fungal pathogen activity in mild, damp soil conditions
Range contraction: the decline is most pronounced in Atlantic-climate western Britain and Ireland, suggesting that the species' western range margin is contracting as winters become milder
Habitat loss: coarsening of grassland habitats reduces the diversity of larval host plants available
Light pollution: as with many nocturnal insects, artificial light may disrupt adult behaviour, navigation, and mating

Despite this, the species remains common across much of continental Europe, Russia, and North America, and its global IUCN status remains Least Concern. The UK decline appears to represent a marginal population phenomenon rather than a global collapse.